Various types of sensors are known, which operate on the basis of providing an output voltage when the composition of the exhaust gases changes from oxidizing to reducing state. One such sensor is described in German Disclosure Document DE-OS No. 27 29 475 to which U.S. Pat. No. 4,155,827 Maurer et al assigned to the assignee of this application, corresponds.
This type of sensor uses a solid electrolyte tube which is closed at one end. A heater element is located in the hollow space within the tube. Manufacture of a closed tube, with a heater element is comparatively difficult and hence expensive. It has also been proposed to use the plate-like solid electrolyte body made of oxygen ion conductive material, in which both electrodes are exposed to the gas to be tested. Such a potentiometric sensor has electrodes having different catalytic action with respect to catalyzing of the gas equilibrium at the surface of the solid electrolyte. Sensors of this type, provide, however, only low output voltage signals, less so than a sensor which uses a reference gas, such as air, separated from the test gas. In automotive applications, the small voltage differences sometimes can be masked by noise or disturbance signals arising within the vehicle, or due to passage of the vehicle close to external disturbing electromagnetic fields, such as high tension transmission lines, railroad catenary systems and the like.